1. Field of the Invention
The present disclosure relates to a device such as a sensor manufactured based on MEMS (Micro Electro Mechanical Systems) techniques, and a method for manufacturing the same. More particularly, the present disclosure relates to a sonic sensor including a diaphragm that detects pressure variations and vibrates in response thereto and that is capable of transducing the vibration into an electric signal, and a method for manufacturing the same.
2. Description of the Background Art
A technique called “MEMS” has recently seen progress in the art, which utilizes miniature processing techniques used in the manufacture of semiconductor LSIs, e.g., techniques for processing silicon. Using the MEMS techniques, various miniature components such as, but not limited to, acceleration sensors, pressure sensors and sonic sensors have been developed and commercialized. Accordingly, there have been demands for applying MEMS to portable devices and downsizing MEMS chips for cost reduction.
In order to detect variations in acceleration or pressure, a sensor using MEMS techniques has a structure with a diaphragm film, which is to be a vibrating member, formed over a silicon substrate having a through hole therein (hereinafter referred to as a “diaphragm structure”).
In one method for forming a through hole in a silicon substrate in order to realize a diaphragm structure, a silicon substrate having a (100) plane as the principal plane is subjected to anisotropic etching with an alkaline etchant such as KOH. This method produces a through hole in the silicon substrate, in which the inner wall surface of the through hole is the Si (111) crystal face being an inclined plane. With a through hole having such an inclined inner wall, the MEMS sensor chip will be large. In other words, the formation of a through hole whose inner wall surface is the Si (111) crystal face becomes a factor that inhibits the downsizing of a MEMS sensor chip.
As a solution to this problem, JP Laid-Open Patent Publication No. 2008-98524 discloses a method for downsizing a MEMS sensor chip by reducing the proportion of the Si (111) crystal face, i.e., the inclined plane, with respect to the area of the MEMS sensor chip.
FIGS. 27A and 27B are a plan view and a cross-sectional view, respectively, showing a conventional diaphragm structure disclosed in JP 2008-98524. As shown in FIGS. 27A and 27B, a conventional diaphragm structure 11 includes an Si substrate 12 having a (100) crystal face as the principal plane, and a device thin film (diaphragm) 13 formed on the Si substrate 12 with a holding section 18 interposed therebetween. The Si substrate 12 includes a through hole 14 running through the substrate 12 from the front surface to the reverse surface thereof, which is formed by wet-etching the Si substrate 12 from the reverse surface. The through hole 14 has a rectangular shape on the principal plane of the Si substrate 12, and the longitudinal and lateral sides of the rectangular shape run along the <110> orientation on the principal plane being the (100) crystal face. The inner wall of the through hole 14 includes inclined planes 15 and 17, each being the (111) crystal face or an equivalent crystal face formed at the front surface side and the reverse surface side of the Si substrate 12, and a vertical plane 16 connecting the inclined plane 15 and the inclined plane 17 with each other. Thus, the cross section of the through hole 14 is tapered at the front surface side and the reverse surface side of the Si substrate 12.